The process of metamorphism involves solid-state chemical reactions among minerals at elevated pressure and temperature deep in Earth's crust, usually associated with the development of mountain belts like the Andes and the Himalayas. The reactions are important in the formation of certain ore deposits and in the distribution of elements in the crust, generally. Some metamorphic reactions release carbon dioxide that ultimately is transported to the atmosphere and hydrosphere. The least understood aspect of metamorphism is its duration. Conventional wisdom holds that regional metamorphism lasts ten million years or more. New, indirect evidence, however, has emerged that the duration could be much shorter, 100,000 years or less. Resolution of this difference is essential to fully understand how metamorphic mineral reactions affect ore formation, element transport in the crust, and global climate through the release of carbon dioxide. Proposed research is to use a newly developed instrument for precise measurement of carbon isotope composition at high spatial resolution within individual crystals of calcite (calcium carbonate) in metamorphic rocks. Variations in carbon isotope composition within calcite crystals can serve as a clock that will more precisely record or set an upper bound on the duration of metamorphism.
Proposed research will involve contact and regionally metamorphosed rocks from California, Maine, Vermont, and Scotland. Samples will be screened using electron imaging techniques at Johns Hopkins University. Analysis of carbon isotope composition will involve several institutions and instruments. Micro-drilling and laser ablation methods with conventional gas source mass spectrometry at the Universities of Maryland and New Mexico will be used to target individual calcite crystals for detailed study. The innovative analytical technique will be high-precision carbon isotope analysis at the 10-micron spatial scale using the newly installed ion microprobe at the University of Wisconsin. Measured profiles in carbon isotope composition within individual calcite crystals will be inverted, using standard mathematical treatments of diffusion and excellent new measurements of diffusion coefficients from Oak Ridge National Laboratory, to estimate the duration of metamorphism. Results will additionally bear on application of the carbon isotope composition of calcite to estimate temperature, fluid composition, and mass transport distances during metamorphism.
